Separation of gas mixtures



Sept. 5, 1966 J. S. KAMLANI SEPARATION OF GAS MIXTURES Filed April 23. 1963 4 34 37 ./-25 h x Y 4 10 CRUDE o -suacooLER HIGH PRESSURE COLUMN NITROGEN SUBCOOLER OXYGEN GAS 2a :DF- 5 PROCESS REBOILER AIR 27\ INL T NITROGEN UI LIQUID GAS g g OXYGEN INVENTOR AT TORNE YS.

United States Patent vIl 3,270,514 SEPARATION OF GAS MIXTURES Jeram S. Kamlani, Fairfield, Conn., assignor to Gas Equipment Engineering Corporation, Milford, Conn. Filed Apr. 23, 1963, Ser. No. 275,009 1 Claim. (Cl. 62-29) This invention relates to separation of gas mixtures. While the invention was occasioned by the need for improvement in separation of air into oxygen and nitrogen, the procedure and the apparatus provided by the invention have general application for separation of gas mixtures which include components differing in properties as do the components of air, so that the separation can be effected by liquefaction and fractionation. Thus, whereas the invention will be described in detail with reference to separation of air into oxygen and nitrogen, it will be understood that the procedure and apparatus of the invention find application in the separation of other gas mixtures.

It is known to separate air into oxygen and nitrogen by a two-step fractionating procedure, the first step be ing a high pressure fractionation and the second step be ing a low pressure fractionation. Thus, air cooled to a low temperature is introduced into a high pressure fractionation column which can be operated at, for example, 75-150 p.s.i.a. to provide a crude oxygen cut as bottoms of the high pressure fractionation and a vapor rich in nitrogen as overhead of the high pressure fractionation. The crude oxygen cut is then transferred to the low pressure column and is there fractionated to provide oxygen of high purity, for example, 99% oxygen. The overhead of the low pressure column, like the overhead of the high pressure column, is nitrogen. It is common to provide the equipment so that the condenser for the high pressure column functions as the reboiler for the low pressure column and to utilize as the cooling fluid for condensation of overhead of the high pressure column, the liquid oxygen product (bottoms) of the low pressure column. Thus, nitrogen for reflux for the high pressure column is provided. Further, by the condensation of the overhead of the high pressure column, liquid nitrogen can be obtained for utilization as reflux in the low pressure column. In general, the principal product of such operation is liquid oxygen. Oxygen gas, nitrogen gas, or liquid nitrogen can also be taken as product, depending on the particular operating conditions employed. In apparatus of the type referred to here, the low pressure column is disposed above the high pressure column and the condenser-reboiler is disposed between the two columns. This apparatus has the disadvantage of requiring very considerable height due to the arrangement of the low pressure column atop the high pressure column.

It has been proposed to reduce the height requirements for an air separation plant by providing a so-called split column, i.e. a plant wherein the high pressure column and the low pressure column are located side by side, rather than one atop the other. The instant invention is directed toward providing an'improved split column operation.

It is an object of the invention to provide a split col umn for air separation wherein the heat transfer area requirements are substantially less than has been the case heretofore.

Another object of the invention is to provide for split column operation wherein the use of mechanical or heat or vapor pumps to transfer material from one column to the other are not required.

Still another object of the invention is a split column operation wherein savings can be realized in respect to 3,270,514 Patented Sept. 6, 1966 the equipment requirements, more particularly in respect to the diameter of the low pressure column.

Other objects of the invention will be apparent from the ensuing description.

According to the invention, a gas mixture including a low boiling component and a high boiling component is separated by liquefaction and fractionation. The mixture is first fractionated in a high pressure column to provide an overhead vapor rich in the low boiling component and as bottoms a crude cut of the high boiling component in liquid state. A portion of the high pressure column overhead vapor is condensed and the resulting c-ondensate is used as reflux for the high pressure column. Crude cut, i.e. bottoms of the high pressure column, is transferred to a low pressure column and fractionated therein to provide a low pressure column overhead vapor rich in the low boiling component and a low pressure column bottoms rich in the high boiling component. In these operations, the process of the invention is akin to known procedures. The process of the invention departs from the prior art in that crude cut, i.e. bottoms of the high pressure column, is reduced in pressure and is then utilized for condensing the reflux for the high pressure column by indirect heat exchange of crude cut at the reduced pressure and the overhead vapor of the high pressure column. Further, an overhead vapor product rich in the low boiling component is withdrawn from the high pressure column and bottoms of the low pressure column are reboiled by indirect heat exchange of the overhead vapor product of the high pressure column and the bottoms of the low pressure column.

A feature of the invention is that the transfer of crude cut from the bottom of the high pressure column to the top of the high pressure column, for utilization thereof in the condenser, can be effected by the pressure of the crude cut at the bottom of the high pressurecolumn, so that it is not necessary to resort to mechanical pumping or other means for effecting the desired lifting of the crude cut. Further, the crude cut utilized for the indirect heat exchange in the condenser of the high pressure column can be transferred to the low pressure column for fractionation, and this transfer can be effected by the pressure of crude cut in the condenser. Thus, the transfer of crude cut to the condenser and from the condenser to the low pressure column can be effected by pressure of the crude cut and without the utilization of means such as pumps. 7

The utilization of crude cut for condensation of over head vapor of the high pressure column provides the advantage that reduced heat transfer area is required in conrparision to prior art procedures wherein there is used for this condensation a substantially purer cut of the high boiling component. This advantage is utilized in the procedure described above, wherein reflux for the high pressure column is condensed by indirect heat exchange with crude cut or bottoms of the high pressure column, and wherein additional overhead vapor product of the high pressure column is removed therefrom as a vapor and trasferred to the low pressure column wherein it .is used as heating medium in the reboiler of the low pressure column. If desired, overhead vapor product of the high pressure column in addition to that required for reflux for the high pressure column can be condensed in the condenser by indirect heat exchange with crude cut, and the condensate in excess of that required for the reflux, can be transferred to the low pressure column. Such excess can be utilized as reflux in the low pressure column.

It is preferred, however, to condense in the condenser atop the high pressure column only part of the overhead vapor product of the high pressure column and to withdraw 07 from the high pressure column vapor for use as heating medium in the reboiler of the low pressure column.

The invention is further described in reference to the accompanying drawing, wherein a how sheet and equipment arrangement according to the invention is indicated.

The apparatus of the invention can include a first or high pressure fractionating column 21 outfitted with an overhead condenser 22. The overhead condenser can be of the dome type common to air separation apparatus and includes the tubes 23 and dome 24. The tubes are in communication with the fractionating column so that overhead vapor from the fractionating column can .pass into the tubes and the dome for indirect heat exchange with material on the outside of the tubes and the dome. Further, the high pressure column 21 is provided with process air inlet line 1 and with conduit 2 which communicates the bottom of the fractionating column with the condenser for transfer of crude cut or bottoms of the fractionating column 21 to the condenser for indirect heat exchange between crude cut and overhead vapor product inside the tubes and dome of the condenser 22.

The apparatus further includes a second or low pressure column 25 which is provided with a reboiler 26. The reboiler 26 can be constructed similar to the condenser of the high pressure column and includes the tubes 27 and the dome 28. Conduit means in the form of line 6 communicate the top of the high pressure column 21 with the tube side of the reboiler 26 for transfer of vapor from the tctp of the high pressure column to the reboiler wherein the vapor is utilized as heating medium for the reboiler. Further, there is provided conduit means in the form of line for transferring crude cut from the shell ide of the condenser 22 to the low pressure column 25. Vapor generated in the condenser 22 passes through line 29 and into line 5, and liquid from the condenser 22 passes through line 30 to line 5, and the vapor and liquid are introduced into the low pressure column 25 at an intermediate point thereof. Some vapor may be present with liquid in line 30.

In the reboiler 26 the overhead vapor utilized for heating condenses and the conduensate passes through line 7 to the sub-cooler 31, then through line 8 and line 9 to the top of low pressure column 25 where the condensate serves as reflux. The overhead vapor from the low pressure column passes through line to the sub-cooler 31 and in the sub-cooler is passed in indirect heat exchange relationship with respect to the condensate flowing to the top of the low pressure column for use as reflux. From the sub-cooler 31, the overhead of the low pressure column flows through line 11 to sub-cooler 32, wherein the vapor ower-head from the low pressure column is passed in indirect heat exchange relationship with the crude cut passed from the bottom of the high pressure column 21 to the condenser of the high pressure column. The vapor leaves the sub-cooler 32 through line '12. Substantially pure high boiling component is withdrawn as liquid from the reboiler via line 14. The high boiling component, substantially pure, in the form of gas, can be withdrawn through line 15. The low boiling component, in liquid state, can be withdrawn through line 13.

The apparatus of the invention is provided with suitable control valves, and can be operated in a manner to provide liquid gas products in selected proportions.

In utilization for separation of air into oxygen and nitrogen, the process air introduced through line 1 can be at a low temperature and can be about 6-25 liquid. The high pressure column can be operated at a pressure in the range of 75150 p.s.i.a. The crude oxygen cut passed through line 2 is reduced in pressure by valve 34 to the pressure utilized on the shell side of the condenser 22. Thi pressure is intermediate the pressure of the high pressure column and the pressure of the low pressure column. The pressure of the low pressure column can be 20-40 p.s.i.a. The vapor draw-off line 29 and the liquid draw-off line are provided, respectively, with valves 35 and 36, and these can be manipulated to provide the desired operation of the condenser 22. Nitrogen gas passed via line 6 to the reboiler 26 is condensed in the reboiler and passed through lines 7, 8, and 9 to the top of the low pressure column. The valve 37 is utilized to control the flow of nitrogen to the top of the low pressure column. The nitrogen gas line 6 connecting the top of the high pressure column 21 with the reboiler 26 of the low pressure column 25, is provided with a draw-off line 33 having valve 38; the liquid nitrogen line 13 is provided with valve 39; the liquid oxygen line 14 is provided with valve 40; these various valves can be manipulated to provide suitable operation of the process.

An advantage of the procedure of the invention is that the portion of the low pres-sure column 25 intermediate the reboiler and the point of introduction of the crude cut of low boiling material, e.g. oxygen, can be of reduced diameter, since the vapor load in this section of the column is less than the vapor load above the point of introduction of the crude cut. For example, the crosssectional area of the column below the point of introduction of the crude cut can be about 5065% of the crosssectional area of the column above the point of crude cut introduction.

The invention is further described in the following example.

Example A plant according to the flow sheet set forth in the drawing was assembled using available equipment. The high pressure column had four GEECO Navy-type perforated trays with a helical coil condenser at the top. The low pressure column had twenty-three GEECO Navy-type perforated trays. The reboiler associated with the low pressure column was a horizontal shell-and-tube type exchange located adjacent to but separate from the column itself. The sub-coolers were wound Helicoil exchangers. The plant was operated to provide a liquid oxygen product of purity 97.6% and a nitrogen vapor product. Neither gaseous oxygen nor liquid nitrogen were taken as product. Conditions for the various streams, measured or calculated, were as is set forth in the following table.

Line Flow, Press, Temp, Comp.

lbs/hr. p.s.i.g. F. percent 0 Figures in parentheses were calculated.

Equipment having more trays can be utilized to provide higher purity oxygen. Thus, by utilizing more trays, oxygen of purity of 99.5% can be produced.

In this example, the temperature of the oxygen product (line 14) is lower than the temperature of the crude oxygen in the condenser 22 (line 4). Normally in the operation of the process of the invention, the temperature of the crude oxygen would be lower, and the relationship is reversed in the example because of equipment limitations applying in the example. Thus, there was excess area in the condenser 22, and it was therefore necessary to increase the pressure on the crude oxygen to reduce temperature difference and force more of the load into the reboiler of the low pressure column.

While the invention has been described in reference to particular embodiments thereof, various modifications will be apparent from the description, and it is desired to secure by these Letters Patent all such variations.

What is claimed is:

Apparatus suitable for separation of a gas mixture including .a low boiling component and a high boiling component by liquefaction and fractionation comprising:

(a) a first fractionating column for fractionating the mixture at a relatively high pressure to provide an overhead vapor rich in the low boiling component and as bottoms a crude cut of the high boiling component in liquid state, a condenser for said first fractionating column for condensing by indirect heat exchange overhead vapor of said first fractionating column for use as reflux in said first fractionating column, means communicating said first fractionating column with said condenser for transfer of overhead vapor to the condenser for condensation, and means communicating the condenser with the said first fractionating column for transfer of condensate to said column for use as reflux therein;

(b) a second fractionating column for fractionation of crude cut produced in said first fractionating column to provide a second fractionating column overhead vapor rich in low boiling component and a second fractionating column bottoms rich in the high boiling component, a reboiler for the bottoms of said second fractionating column for reboiling of said bottoms by indirect heat exchange;

(c) conduit means interconnecting the bottom of the said first fractionating column with the condenser of the first fractionating column for transfer of first fractionating column bottoms to the condenser for indirect heat exchange with the overhead vapor transferred to the condenser;

(d) conduit means interconnecting said first fractionating column and said reboiler of the second fractionating column for transfer of overhead vapor of the first fractionating column to said reboiler for indirect heat exchange between said overhead vapor of the first fractionating column and bottoms in said second fractionating column;

(e) conduit means interconnecting the condenser of the first fractionating column and the second fractionating column for transfer of crude cut used in the condenser for condensing overhead vapor of the first fractionating column to the second fractionating column for the fractionation thereof in the second fractionation column;

(f) means for supplying low boiling component in liquid state to the top of said second fractionating column for use as reflux in said second fractionating column;

(g) said conduit means interconnecting the bottom of the first fractionating column with the condenser being outfitted with a pressure reducing means for reducing the pressure of crude cut transferred through said conduit;

(h) said means for supplying reflux to the second fractionating column comprising conduit means interconnecting the reboiler of the second fractionating column with the top of said second fractionating column for transfer of liquid formed in the reboiler by condensation of the first fractionating column overhead vapor utilized therein, to the top of the second fractionating column, whereby said liquid is provided as reflux for the second fractionating column;

(i) said conduit means for transferring crude cut from the first fractionating column condenser to the second fractionating column being connected to the second fractionating column for introduction of said crude cut at an intermediate point over the height of the second fractionating column, the diameter of the second fractionating column below said intermediate point being less than the diameter of said second fractionating column above said intermediate point.

References Cited by the Examiner UNITED STATES PATENTS NORMAN YUDKOFF, Primary Examiner. 

